Electrostatic clamps are used in the semiconductor industry to firmly hold a silicon wafer while various processes are performed on the wafer. Relative to mechanical clamps, electrostatic clamps have significant advantages, which include (a) an increased ability to transfer heat, (b) a reduction in mechanical wear on the wafer, which results is chipping and other mechanical defects, (c) an increase in the effective area of the wafer that may be used to produce saleable products, (d) a decrease in the number of particulates generated, (e) reduced contamination of the clamp from the ion beam used in sputtering, and (f) uniformity of the clamping force across the surface of the wafer.
The semiconductor industry is not the only industry which uses electrostatic clamps. For example, several LCD (liquid crystal display) manufacturers use electrostatic clamping techniques to hold special glass during processing. The solar cell industry also uses electrostatic clamps.
An electrostatic clamp holds a work piece (e.g. semiconductor wafer, glass or other object being worked on) by creating a capacitor. In order that the work piece can be held to the electrostatic clamp, either the work piece is conductive, or a conductive plating is applied to the work piece before clamping. In a simple electrostatic clamp, the work piece becomes an electrode of the capacitor and the clamp provides the other electrode. If the clamp has only a single electrode, then the work piece must have an electrical connection to ground, typically via a conductor or ionized gas. When the clamp electrodes are charged, the work piece becomes oppositely charged in the area of the electrode, and is attracted to the clamp electrode. The clamping force can be calculated using Coulomb's law.
The electrostatic clamp provides a thin layer of material between the clamp electrodes and the work piece. In this document, the material provided by the electrostatic clamp that resides between the clamp electrode(s) and the work piece is called the “barrier material”. Typically, the thickness of the barrier material is on the order of tens of microns. Depending on the electrostatic clamp technology, the barrier material can be either a pure dielectric (in the case of a Coulombic clamp) or a semi-insulative material (in the case of a Johnsen-Rahbek clamp).
In more complex electrostatic clamps, the clamp provides more than one electrode. In the case of a clamp that has two electrodes (a.k.a. a bi-polar clamp), the charge on a first one of the clamp electrodes is opposite polarity to the charge on a second one of the electrodes. This arrangement forms a capacitance from one clamp electrode, through the barrier material, to the work piece, back through the barrier material and then to the other clamp electrode. Electrostatic clamps having more than two electrodes are a variation on the bi-polar clamp, but operate in a manner that is similar to the bi-polar clamp.
One of the issues with electrostatic clamps is the length of time it takes for the work piece to “declamp”. Declamping is a process where the work piece is released from the electrostatic clamp, and the work piece is then moved to its next processing station. Declamping can be hindered by a built up residual static charge, which prevents release of the work piece.
Another issue results from the thickness of the barrier material, which is located between the work piece and the electrodes. The barrier material is very thin, and can be damaged easily. Damage to the barrier material may result in detectable current flowing between the electrode and the work piece. Such a fault results in reduced force holding the work piece to the clamp, which may cause the work piece to move during processing, or the work piece may leave the clamp surface altogether. Also, arcing may occur between the work piece and the electrodes, and such arcing can damage the work piece.
Additional issues arise which are not directly related to the electrostatic clamp itself. For example, if a work piece is damaged, the quality of clamping will suffer. In the event that a work piece is seriously warped, clamping may not flatten the work piece enough, thereby resulting in an imperfect or non-existent force between the work piece and the clamp. Additionally, in the event that an open circuit is created, for example due to either electrical or mechanical stress in the circuitry between the high voltage supply and the electrostatic clamp, then the high voltage may not appear on the electrodes, and the work piece will be partially or completely unclamped.
To solve some or all of these issues, a tool is needed that can be used to detect errors and optimize operational parameters of an electrostatic clamp. Such a tool would allow scientists and engineers to monitor leakage currents, work piece voltages and capacitances associated with electrostatic clamping, while at the same time, allowing unique waveforms for clamping, work piece processing, and de-clamping to be output to the electrostatic clamp. Currently, electrostatic clamp manufacturers, power supply designers, and end users tend to work independently to create a system that works, and such a tool would allow flexibility amongst these groups to create a robust system that optimizes not only the process flow, but safety design margins as well.